Experimental Study on Frequency Modulation Damping Effect of Digital Detonator in Road Clearance Blasting

. To measure and evaluate the impact of vibration on the surrounding buildings and structures during highway rock burst removal and to formulate scientifc and reasonable protective technical measures, the authors used the blasting perilous rock removal project of the K2227+920–K2228+000 section of the Shaanxi G316 road as a case study. Te application of frequency modulation blasting vibration damping technology adopting digital electronic detonators was investigated. Te diference in vibration peak and frequency between the perforation-by-hole initiation of the detonator and the frequency modulation initiation of the digital electronic detonator is compared. Te results showed that by adopting an accurate delay of digital electronic detonators and the damping scheme of adjusting the blasting vibration frequency using the electronic detonators a reduction in blasting vibration velocity can be achieved. For an unchanged blasting scale, hole mesh parameters, and explosive unit consumption, the total blasting time was adjusted in the test. Compared to the hole-by-hole initiation, the blasting vibration velocity at a pier of the G85 Baohan highway bridge located 52m away was reduced from 0.367cm/s to 0.229cm/s, a decrease of 36.7%. Te feld test demonstrated that frequency modulation and vibration attenuation using digital detonators can reduce blasting vibrations and efectively reduce the infuence of blasting construction on surrounding buildings and structures.


Introduction
In recent years, numerous blasting accidents occurred during road clearance activities that caused signifcant economic losses and received bad press. Blasting in complex environmental conditions has become a challenging problem restricting engineering construction [1,2]. Te main risks to surrounding environment involved in road clearing blasting operations include vibrations, fying rocks, and air shock waves [3][4][5].
At present, there is a lack of systematic research on the infuence of blasting operations carried out during road construction activities under complex environmental conditions on the surrounding environment [6][7][8]. Terefore, this study aims at closing this gap in knowledge. Blasting risks can be controlled and preempted depending on the specifc environmental conditions encountered during construction, ensuring the safety of road clearing operations and reducing the interference with the surrounding environment. Under the same blasting vibration intensities, the lower the main vibration frequency, the closer it is to the selfvibration frequency of the protective object, and the greater the possibility of causing damage to the building (structure) [9,10]. Terefore, by reducing the maximum detonation dosage of a single sound and shortening the delay time, the main vibration frequency can be efectively increased, so as to improve the safety of blasting vibration. If the delay between two holes is d, then the vibration spectrum of millisecond blasting has an obvious peak at the frequency of 1/d interval; by changing the delay interval, the frequency component of the blasting vibration can be changed, thus avoiding the self-vibration frequency band of the construction structure.
Te use of a digital detonator makes the group holes detonate continuously at a very small jet lag diference, which is equivalent to adding blasting with a high base frequency as a forced vibration source, which causes the main vibration frequency in the near zone of blasting to tend to the base frequency of the vibration source, greatly improves the main vibration frequency, and reduces the possibility of resonance of building (construct).
Tis study adopted the K2227+920-K2228+000 perilous rock blast clearing project in the G316 road section in Shaanxi province as a case study. A damping scheme of the electronic detonator adjusting the actual blasting vibration frequency (frequency modulated damping) was studied with the aim of reducing blasting vibration velocity [11][12][13]. Using feld tests, it was demonstrated that digital detonators can reduce blasting vibrations and protect the safety of buildings and other structures adjacent to the blasting site. Tis research will serve as a practical reference for scientifc investigations and quantitative engineering design of road blasting operations in similar complex environmental conditions.

Mechanism of the Electronic Detonator Priming System.
Te height of the building (above ground) exerts some amplifcation efect on the vibration velocity of blasting particles (in the horizontal direction), but the main frequency of blasting seismic waves has a signifcant impact on this amplifcation efect. Under the same conditions of blasting vibration intensity, the lower the main frequency, the closer it is to the natural frequency of the protected structure, and the higher the amplifcation rate of the particle vibration velocity, the greater the likelihood of causing damage to buildings (structures) [4,5]. Methods such as reducing the maximum amount of initiating explosive per shot and shortening the delay time can efectively increase the main vibration frequency, thereby improving the safety of blasting operations.
Te dependence of frequency on distance generally exhibits a negative exponential change, and due to the highfrequency fltering efect of soil, there will be a rapid decrease for high frequencies. Due to the fact that the lowfrequency blasting vibration waves are close to the natural frequency of rock media, there will be a gentle decrease in low frequencies in the far-feld region. Te fundamental frequency of buildings is 3-10 Hz, derived from a shear cantilever model with a constant cross-section, and, based on the measured results, the following empirical formula for the natural vibration period of buildings (structures) is recommended: where T is the basic period, s, and H 0 is the calculation height of buildings (structures), m. After the building (structure) is damaged by blasting vibrations, the natural vibration period will signifcantly increase in step with the severity of damage. Statistical data analysis shows that the period of a building (structure) after cracking may increase by 20% to 60% compared to the original structure.
Digital detonator frequency modulation shock absorption refers to the change in the total duration, intrahole delay, and interhole delay of blasting, which can be arbitrarily and accurately adjusted by using digital detonators placed in multiple blasting holes together with considering the actual rock mass engineering conditions. Te vibration frequency of blasting and the diference in resonance frequency compared to the surrounding rock mass and buildings can be increased, while the cumulative efect of the main vibration phases of each group of seismic waves generated by each blasting hole charge is disrupted, to achieve dynamic shock absorption.
If the delay between each two blasting holes is d, then the vibration spectrum of millisecond blasting has a signifcant peak at every 1/d frequency interval; by changing the delay interval, the frequency component of blasting vibration can be changed, thereby avoiding the natural vibration frequency band of buildings (structures). By using digital detonators, when a group of blasting holes are continuously detonated with a small time diference, it is equivalent to adding a higher fundamental frequency blasting as a forced vibration source, resulting in the main vibration frequency of the blasting near the area tending towards the fundamental frequency of the vibration source, greatly increasing the main vibration frequency, reducing the possibility of resonance of building (structure) protective structures, and reducing the harm of blasting vibration.

Principle of Frequency Modulation Shock Absorption.
Frequency modulation shock absorption using digital electronic detonators refers to multiple hole initiation according to actual engineering rock mass conditions. By changing the total blasting time, the initiation delay within a hole, and the delays between multiple holes, the blasting vibration frequency can be enhanced, increasing the gap between it and the resonance frequencies of the surrounding rock mass, buildings, and other structures. Te coherence between the main vibration phases of the group of seismic waves generated by each blasting hole charge is disturbed, and dynamic shock absorption is achieved.
Compared to a large number of studies on blasting vibration particle peak velocity, only limited results on blasting vibration frequency are available [14][15][16][17].
Te main frequency of blasting vibrations changes as the shock waves are attenuated away from the blasting center. To optimize the digital electronic detonator frequency modulation, the average frequency is adopted as the basis of engineering design. Te defnition of average frequency is as follows [18,19]: where f c is the average frequency and A i is the vibration velocity amplitude corresponding to frequency f i .

Shock and Vibration
Experiments showed that the attenuation of the average frequency of blasting vibration is more regular than that of the main frequency. An inverse power function was used to ft the data, and the correlation coefcient was close to 1, showing a signifcant correlation [20][21][22][23][24].

Project Overview.
On February 17, 2018, the upper slope on the left-hand side of the G316 road in Wulipu Village (K2227+920-K2228+000), Wuguanyi Town, Liuba County suddenly collapsed. Te collapsed material spilled onto the entire road surface and a part of it rushed into the nearby river. At present, the upper part of the slope is isolated, and the bedrock is overhanging, which poses a serious hazard to trafc safety and a threat to the substructure of a bridge located in the river on the right side of the highway along the G85 Baohan expressway.
Te outcrop layer of this section of slope is Mesozoic granite. Te rock mass is gray-yellow with a coarse crystal structure, massive structure, and well-developed joints and fssures. Te mineral composition includes feldspar, quartz, and mica. Te rock is relatively hard, and its integrity is relatively uncompromising.
Te slope strata dip is 300°, and the dip angle is 45°. Tere are two main groups of joints: 140°∠85°: 2 pieces/m and 265°∠80°: 1 piece/m. Te joints and strata cut the rock into blocks, which opened to form fssures after unloading.
Te intended perilous rock blast clearance area is shown in Figure 1. It is about 8 m higher than the G316 road surface at Wulipu Village, Wuguanyi Town, Liuba County (K2227+920-K2228+000). Along the north side of G316 is the Bao River. According to the feld investigations, the north side of the blasting construction area is 52 m away from the G85 Baohan Expressway bridge. Tere are residential houses on the west side of the intended perilous rock blast clearance area at a distance of at least 50 m. Six blasting vibration measurement points, denoted #1-#6, were arranged. Measurement points #1-#4 were arranged on the G316 road surface and measurement points #5 and #6 at the foundations of the adjacent expressway bridge piers, as shown in Figure 1.
On March 9, 2018, the Shaanxi Provincial Transportation Planning and Design Institute surveyed the site. Te survey determined that the width of the slope is about 80 m and its height is about 60 m. Tere is still a large volume of perilous rock on the slope, and the crack width exceeds 20 cm. Te collapse caused the upper part of the hill to lose support so that it now forms an overhang. Te combined volume of slope perilous rock and hilltop loose rock is about 1200 m 3 . Te length of the adjacent rock mass is about 70 m, the width is 20-40 m, the thickness is 2-8 m, and the volume is about 1.47 × 10 4 m 3 . Te slope rock mass develops along stratifed structural planes and opens inside the layer, forming an inverted slope. It is afected by gravity, rainfall, and heavy vehicle trafc vibrations, and there is a risk of further collapse of rock strata and joints.
According to the construction drawings of G316 Road Section K2227+920-K2228+000 Major Water Damage Repair Project compiled by the Shaanxi Transportation Planning and Design Institute in April 2020, the right hand side overhanging rock (route K2227+955-K2227+985 left) along the fracture surface (rock joint fracture inclination 45°) was cleared by blasting. Te clearing range was from the left side to the boundary of the collapsed slope, from the right side to the gully, from the lower side to the road surface about 8 m high, and from the top to the top of the hill or no inverted slope. Te maximum clearance thickness was 9.5 m, and the total clearance volume was about 7000 m 3 , as shown in Figure 2 for a photographic view.

Blasting Test Program
In the test, the blasting vibration monitoring data of the frst and second blasting constructions in the blasting clearance area were selected for analysis. In the frst experiment, the detonators initiated hole-by-hole, whereas in the second experiment, a digital electronic detonator frequency modulation vibration reduction plan was implemented, as shown in Figure 1.

Analysis of Blasting Conditions and Experimental Design.
Te main hazards related to blasting operations include blasting vibrations, fying rocks, harmful gases, blasting shock waves, noise, and dust [3]. Considering the surrounding environment of this blasting project, the main risk was deemed to be blasting vibrations.

Analysis of Possible Hazards Caused by Blasting Vibrations
① Te main structure at risk of damage due to blasting vibrations in this project was the G85 Baohan Expressway bridge, which was 52 m away. ② After the explosive material is detonated, a generated stress wave will propagate through the rock or soil layer. When the local seismic wave intensity is greater than the strength of rock mass, it will cause damage and cracking [25][26][27]. Blasting damage may gradually accumulate [28], until the pier foundation of G85 Baohan Expressway bridge eventually becomes unstable. ③ Blasting vibrations can cause the opening, extension and transfxion of original cracks in the rock subgrade slope [15]. As a result, large rock fragments may roll down the slope and impact the G85 Baohan Expressway bridge piers.
If the infuence of the blasting vibrations exceed the safe thresholds, they will cause the foundation and walls of the nearby residential homes located 50 m away to crack and potentially fall [3,29].

Design Principles.
Te blasting construction requirements are demanding and the surrounding conditions are complex. To ensure safe and efcient blasting, the digital electronic detonator hole-by-hole initiation system was implemented to achieve frequency modulation shock absorption, by precise control of explosive energy release, medium movement, crushing and damage accumulation [30], and reduce the amount of fying rocks [31].

Blasting Parameters Design.
Te adjacent rock mass is about 70 m long, 20-40 m wide, 2-8 m thick, and its dip angle is about 45°. According to the principles of hole layout, 70 mm diameter vertical holes in a triangular layout were adopted in the blasting area [32][33][34][35]; see Figure 3.
Te charge holes were perpendicular to the surface of the rock mass, hole spacing was A (m) × row spacing B (m) � 1.0 m × 1.0 m, resistance line W � 1.0 m, west control resistance line was ≥1.2 m, and east resistance line was ≤1.0 m. Considering the perilous rock thickness of 2-8 m, the drilling was carried out to leave a 0.3 m thick protective layer from the bottom of the hole to the main joint slide surface [36]. Te average unit explosive consumption was less than 0.55 kg/m 3 . Te blasting parameters designed for the hole depth of 6.0 mare shown in Table 1.

Detonation Tubes Detonated Hole-by-Hole.
Considering the rock mass properties, the delay time between holes was selected as 25 ms (MS2), the delay time between rows as 75 ms (MS4), and the delay time of charge in holes as 460 ms (MS11). Because the actual and the nominal delay of detonator have large discreteness [37][38][39], there is normally a delay error. Terefore, in the actual initiation process, a hole-by-hole initiation with a small delay time was adopted [40][41][42][43]. Te time settings of hole initiation sequence are shown in Figure 4.

Digital Electronic Detonator Frequency Modulation
Initiation. Tis scheme adopted hole-by-hole initiation combined with a millisecond delay within each hole. Considering the properties of rock mass, the delay between holes was assumed as 6 ms, the delay between rows as 27 ms, and the delay between charge packets in a hole as 5 ms. Te setting of hole delay 6 ms and row delay 27 ms was determined according to the crushing efect of hole distance 1.0 m and row distance 1.0 m throwing efect test. Te electronic detonator time settings of the uppermost charge pack in each hole are shown in Figure 4.
If the amount of charge in a hole exceeded 2.0 kg, an interval charge was adopted [21] by dividing the 2.0 kg charge pack into sections. Te drug package was flled at a 1.0 m interval, and the delay from the orifce to the bottom of the hole was 5 ms.

Layout of Measurement Points. Te EXP3850 blasting vibration instrument manufactured by Chengdu Zhongke
Dynamic Instrument Co., Ltd. was adopted in this study together with velocity sensors, which collected, stored, and analyzed data from the tangential, vertical, and radial   channels. Te EXP3850 blasting vibration recorder is a portable instrument that can record and analyze the seismic waveform signals caused by blasting [44]. Te instrument was directly connected to the velocity sensors and placed near each measurement point. Te instrument can automatically record eight vibration events and their occurrence times and pass data through an RS232 interface to a computer for data processing. Te arrangement of measurement points plays a very important role in blasting vibration testing, because it directly afects the observed data [45,46]. Te number and location of measurement points are determined mainly according to the purpose of testing and feld conditions. Te arrangement of measurement points should follow the following principles [47]: (1) To determine the impact of blasting vibrations on the expressway bridge piers and residential house foundation, representative measurement locations should be selected. (2) Te selected test site has uniform geological conditions and essentially the same lithology. Near a large fault zone, the measurement points should be arranged on one side of the fault or fracture zone. (3) Te observed data can refect the variation in seismic wave characteristics.
Te measurement points in this experiment were arranged on the highway bridge piers and highway foundation, as shown in Figure 5. Te relative positions of measurement points are shown in Table 2.

Vibration Data Analysis.
Te vibration velocity of a rock-soil medium can be decomposed into three mutually perpendicular directions:vertical, horizontal radial, and horizontal tangential [3]. A vector synthesis waveform in three directions was shown as Figure 6.
Te spectrum and power spectrum wave forms in the X, Y, and Z directions of measuring point 3 with moderate location and blasting vibration velocity were selected for analysis and explanation. Te monitoring data are shown in Figure 7.
From the spectrum and power spectrum in the three directions of X, Y, Z of measurement point 3, it can be seen that the energy is mainly concentrated in the frequency band of 20∼30 Hz and 30∼60 Hz only has less energy. Moreover, the power spectrum shows that the energy of the blasting seismic wave is concentrated in the lower frequency band and is mainly concentrated in the 20∼25 Hz frequency band, as shown in Figure 7. And also from the power spectrum in the Z direction, it can be seen that the energy of the signal develops towards high frequencies and energy distribution widens. In addition, there are many subfrequency bands in the range of 25∼100 Hz, and the energy peaks of each subband are not much diferent.
It can be seen from the original power spectrum that the energy frequency of the blasting vibration signal is mainly a low-frequency signal below 30 Hz, and a small part of the other part is 40∼100 Hz. Te main reason is related to the blasting vibration signal propagation medium in the construction area, and the high-frequency signal is easy to flter out or attenuate in the formation, and on the other hand, the wave guide efect and the layered formation can propagate the Z-direction frequency farther.
Te half-wave frequency corresponding to the peak of blasting vibration is called the apparent dominant frequency, which can better refect the frequency characteristics of blasting vibrations [19]. As shown in Figure 8, the apparent dominant frequency of blasting vibration at each measurement point increased with the horizontal distance from the blasting center, and the variations in the apparent dominant frequency in the three directions show a general attenuation trend. Table 3 shows the vibration Shock and Vibration 5 monitoring statistics with adjusted total blasting time, intrahole delay, and interhole delay by adopting the digital electronic detonator initiation system. Te main vibration frequency was between 30 Hz and 60 Hz. Te average frequency in the three directions when the digital electronic detonator initiation system was used was 42.62 Hz, which was nearly 41% higher than the average frequency of 30.28 Hz in the three directions of the hole-by-hole initiation system [48].
According to the above monitoring data of blasting vibration velocity and feld investigation, the Shaanxi G316 Liuba section K2227+920-K2228+000 perilous rock blasting removal project near the G85 Baohan Expressway bridge pier area did not have adverse efects on surrounding buildings and structures. Te experimental results show that using the digital electronic detonator frequency modulated initiation system can reduce the blasting vibration velocity and ensure safe construction. [49].            Figure 9: Comparison of peak vibration velocities between hole-by-hole initiation and frequency-modulated initiation using digital electronic detonators. Note: particle vibration velocity shown is the maximum velocity in Z.

Shock and Vibration
Te on-site blasting vibration monitoring results (see Tables 3 and 4) show that the blasting particle vibration peak velocities at all measurement points in all three directions were always less than the allowable threshold of 1.50 cm/s stipulated in blasting safety regulations GB6722-2014.
As shown in Figures 8 and 9, the dominant vibration frequency was between 20 Hz and 60 Hz [50,51]. At measurement point #6 (placed at bridge pier foundations), the maximum frequency-modulated vibration velocity when the digital electronic detonator initiation system was used was 0.2294 cm/s. On the other hand, the maximum vibration velocity when the hole-by-hole detonation scheme was adopted was 0.3666 cm/s, thus, a reduction of 36.7% was achieved.

Conclusions
Tis paper used the Shaanxi G316 Road section K2227+920-K2228+000 perilous rock blast clearance project to investigate the application of digital electronic detonator frequency modulation shock absorption blasting technology. Te in-situ test results showed that by using a precise digital electronic detonator delay scheme to adjust the actual vibration frequency of blasting vibrations (frequency modulated damping), it is possible to achieve a reduction in blasting vibration particle velocity. For unchanged blasting scale, hole array parameters and explosive unit consumption, by adjusting the total blasting time, digital detonator frequency modulated shock absorption can reduce blasting vibrations and guarantee better safety of surrounding buildings and structures compared to the traditional hole-by-hole initiation scheme.
Te conclusions and recommendations of this research are as follows: (1) Based on the resonant frequency of buildings and structures, the response frequency of blasting vibration, and the attenuation law of low-frequency blast wave propagation, the delay time between blastholes is adjusted using digital detonators to change the blasting vibration frequency, so as to achieve vibration reduction, which provides a basis for safe planning of road blasting construction projects under complex environmental conditions. (2) By utilizing the high-precision and arbitrarily delayed time characteristics of digital detonators, a 5 ms in-hole delay is used to form a 3D in-hole charge initiation sequence through the joint action of the surface initiation network and in-hole initiation network during the initiation process of blasting. Experimental results have shown that good crushing and shock absorption efects can be achieved. (3) According to the in situ monitoring data, the blasting interval between holes was reduced from 50 ms to 9 ms, the initiation time of the whole explosion area was reduced from 250 ms to 118 ms, the vibration frequency increased from 20 Hz to 40 Hz, and the intensity of the explosion source was reduced from 4.2 kg to 2.2 kg. At the pier of the G85 Baohan Expressway bridge located 52 m north from the blasting area, the blasting vibration velocity decreased from 0.367 cm/s to 0.229 cm/s, i.e., by 36.7%. (4) Te diference in timing accuracy between the digital detonator initiation system and the detonating network of the nonel detonator during the initiation process results in a change in the superposition of the residual stress of the frst detonating charge and the explosive stress of the second detonating charge. Te respective rock-breaking mechanisms are also different. If the blasting conditions are fxed, the digital detonator initiation system and the detonating network use the same initiation sequence. Due to the large timing error, the detonating network of the detonating tube disrupts the stress feld, and the seismic efect of blasting is smaller than that of blasting using the digital detonator initiation system. Terefore, in the design and execution of blasting, the design rules of the detonator initiation network cannot be simply applied to the digital detonator initiation system.

Data Availability
Te data used to support the fndings of the study are available within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest.